Sheathed-element glow plugs, which are used in combustion engines for igniting a fuel-air mixture, are preheated in the cold state until their temperature is high enough to ignite the fuel-air mixture. For that purpose, the sheathed-element glow plug has a heating element which applies an elevated heating voltage to the cold sheathed-element glow plug within a brief time period of one to two seconds, thereby overloading the sheathed-element glow plug at that point in time. Upon completion of this so-called push phase, the tip of the sheathed-element glow plug reaches a temperature of over 1000° C., while the temperature of the remaining portion of the sheathed-element glow plug is still far below this 1000° C. temperature.
The sheathed-element glow plug is normally regulated or controlled by measuring the resistance of a glow filament therein. Since upon completion of the push phase, the remaining portion of the sheathed-element glow plug and thus also the remaining portion of the glow filament have not yet reached the temperature of the tip of the sheathed-element glow plug, a normal temperature regulation or control cannot be performed by measuring the resistance of the glow filament. The transient thermal response, which arises in the sheathed-element glow plug following the push phase, lasts approximately 30 seconds. Subsequently thereto, the temperature in the sheathed-element glow plug equalizes, so that a normal temperature regulation or control can be performed as a function of the measured resistance.